In an internal combustion engine using gasoline and an alcohol interchangeably or together in a mixed state, an alcohol sensor is disposed to detect the alcohol concentration and the fuel feed rate is controlled based on the detected alcohol concentration.
As in an internal combustion engine using a single fuel, under predetermined conditions, by an O.sub.2 sensor, it is judged whether the air-fuel ratio is rich or lean as compared with the target air-fuel ratio (theoretical air-fuel ratio), and the feedback control is performed so that when the air-fuel ratio is reversed from a lean level to a rich level or vice versa, an air-fuel ratio correction coefficient for correcting the basic fuel feed rate is decreased or increased by a proportion constant and the air-fuel ratio is gradually decreased or increased by an integration constant until the air-fuel ratio is reversed again, whereby the air-fuel ratio is brought close to the target air-fuel ratio (see Japanese Unexamined Patent Publication No. 56-98540). Incidentally, as a simple control method, there is known a method in which the air-fuel ratio control is performed only by the integration constant.
Gasoline is different from the alcohol in the absorption reactivity to a ternary catalyst, and this difference results in the following differences of the characteristics at the above-mentioned air-fuel ratio feedback control.
Gasoline is different from the alcohol in the absorption reactivity to a ternary catalyst, and this difference results in the following differences of the characteristics at the above-mentioned air-fuel ratio feedback control.
More specifically, when the comparison is made in the combustion of the same air-fuel ratio, the O.sub.2 concentration in a combustion gas of the alcohol is lower (the amount of the portion left in the form of H.sub.2 O is larger than the amount of the portion left in the form of O.sub.2) than the O.sub.2 concentration in a combustion gas of gasoline, and therefore, the amount of the O component retained in the ternary catalyst is lower and when the fuel injection quantity is increased, the oxidation of CO, HC and the like increased in the exhaust gas by the O component in the ternary catalyst is completed in a shorter time than in case of gasoline. In short, the exhaust amounts of CO and HC increase promptly in correspondence to thickening of the air-fuel ratio.
Accordingly, if the integration constant at the above-mentioned air-fuel ratio feedback control is set at a value suitable for the use of a gasoline fuel, the deflection of the air-fuel ratio becomes too large when an alcohol fuel is used, with the result that the exhaust amounts of CO and HC increase.
Therefore, in the case where the alcohol concentration is high, by reducing the integration constant, the deflection of the air-fuel ratio at the air-fuel ratio feedback control in the stationary state can be reduced and hence, the exhaust amounts of CO and HC can be reduced.
However, if the integration constant is merely reduced as mentioned above when the alcohol concentration is high, the response characteristic of the air-fuel ratio of the air-fuel mixture is reduced, at the transient driving, and therefore, the air-fuel ratio becomes excessively rich or lean to increase the exhaust amounts of CO, HC and NOx.
The present invention has been completed under this background, and it is a primary object of the present invention to reduce the exhaust amounts of pollutants such as CO and HC by making the deflection of the air-fuel ratio during the air-fuel ratio feedback control in a stationary state constant irrespectively of the fuel concentration.
Another object of the present invention is to maintain a good response characteristic during the air-fuel ratio feedback control in a transient state and prevent the air-fuel ratio from becoming too rich or too lean while keeping a good transient state, and to prevent the increase of the exhaust amounts of pollutants such as CO, HC and NOx.
Still another object of the present invention is to attain the abovementioned effects only by software processing without increase of the cost.
A further object of the present invention is to provide an air-fuel control apparatus especially suitable for an internal combustion engine in which an alcohol and gasoline are used interchangeably or together in a mixed state.